Field of the Invention
The present invention relates generally to switched electrical devices. More particularly, the present invention is directed to self-testing circuit interrupting devices, such as ground fault circuit interrupter (GFCI) devices, that switch to a “tripped” or unlatched state from a “reset” or latched state when one or more conditions is detected. Such devices consistent with the invention disclosed herein have more robust self-testing capabilities than provided in previously known GFCI devices.
Description of Related Art
GFCI devices having contacts that are biased toward the open position require a latching mechanism for setting and holding the contacts in a closed position. Likewise, switched electrical devices having contacts that are biased toward the closed position require a latching mechanism for setting and holding the contacts in an open position. Examples of conventional types of devices include devices of the circuit interrupting type, such as circuit breakers, arc fault interrupters and GFCIs, to name a few.
To be commercially sold in the United States a GFCI device must conform to standards established by the Underwriter's Laboratory (UL) in conjunction with industry-leading manufacturers as well as other industry members, such as various safety groups. One UL standard covering GFCI devices is UL-943, titled “Standard for Safety—Ground Fault Circuit Interrupters.” UL-943 applies to Class A, single- and three-phase, GFCIs intended for protection of personnel and includes minimum requirements for the function, construction, performance, and markings of such GFCI devices. UL-943 requires, among other things, specific fault current levels and response timing requirements at which the GFCI device should trip. Typically, GFCIs are required to trip when a ground fault having a level higher than 5 milliamps (mA) is detected. Further, when a high resistance ground fault is applied to the device, the present version of UL-943 specifies that the device should trip and prevent current from being delivered to the load in accordance with the equation, T=(20/I)1.43, where T refers to time and is expressed in seconds and I refers to electrical current and is expressed in milliamps. Thus, in the case of a 5 mA fault, the device must detect the fault and trip in 7.26 seconds or less.
With such safety-related standards in place, and because GFCI devices are directly credited with saving many lives since their introduction in the early 1970s, they have become ubiquitous throughout the residential and commercial electrical power grid. Like most electro-mechanical devices, however, GFCI devices are susceptible to failure. For example, one or more of the electronic components that drive the mechanical current interrupter device can short-out or otherwise become defective, as can components in the fault detector circuit or elsewhere within the device, rendering the device unable to properly detect the ground fault and/or properly interrupt the flow of electrical current. For this reason it has long been required that GFCI devices be provided with a supervisory circuit that enables manual testing of the ability of the device to trip when a fault is encountered. Such supervisory circuits are typically have a TEST button which, when pressed, actuates a simulated ground fault on the hot and neutral conductors. If the device is functioning properly the simulated fault is detected and the device will trip, i.e., the mechanical interrupter is actuated to open the current path connecting the line side of the device, e.g., where the in AC power is supplied, and load side, where the user connects his or her electrical appliance, etc. and where downstream receptacles or additional GFCI devices are connected.
A study performed by industry safety groups indicated that most often the public does not regularly test their GFCI devices for proper operation, i.e., by pressing the TEST button. This study further revealed that some GFCI devices that had been in service for an extended period of time became non-functional and were unable to properly detect a fault condition, thus, rendering the device unsafe. Specifically, it was discovered that after extended use GFCI devices fail to trip when a fault occurs, thus rendering the device operable as an electrical receptacle but unsafe in the presence of a fault condition. Because the devices are not being regularly tested, this unsafe condition is exacerbated. That is, people falsely believe the device is operational, in view of the fact that it adequately delivers power, when in fact the device is a potentially life-threatening hazard.
The discovery that GFCI devices deployed in the field are becoming increasingly non-operational and unsafe in combination with the realization that people do not regularly test their GFCI devices, regardless of manufacturer's explicit instructions to do so, initiated investigations into possible changes to the UL-943 standard to require the GFCI devices to self-test (e.g., auto-monitor) themselves without the need for human intervention. The changes contemplated to UL-943 further included a requirement for either a warning to the consumer of the loss of protection and/or the device automatic removing itself from service, e.g., permanently tripping. Moreover, these additional self-testing operations would have to be performed without interfering with the primary function of the device, i.e., tripping when an actual fault was encountered.
The revised self-test functionality mentioned above is not yet a requirement for UL-943 certification, but it is expected that it will be soon. In preparation for this significant UL change, and in view of the seemingly endless reduction in the cost of integrated circuits, many GFCI manufacturers have migrated to digital techniques (e.g., microprocessors and microcontrollers) in favor of previous analog designs to provide both ground fault protection and self-monitoring functionality. The digital solutions offered thus far, however, are not ideal. For example, several related art GFCI designs, including those directed at providing self-test functionality, suffer from nuisance tripping, a situation where the interrupter is actuated when neither a real ground fault, a manually generated simulated ground fault, nor an automatic self-test fault are present. This unfavorable condition is believed by many to be worsened by the additional requirement of automatic self-testing, which results in additional inductive currents being generated within the device.
It is therefore desired to provide a GFCI device that provides certain self-testing capabilities, including those proposed in the next revision of UL-943, but minimizes the risks associated with nuisance tripping.